a) Field of the Invention
The present invention relates to improved methods of kneading a powder of metal, ceramic, cermet, or other materials with a binder, molding the kneaded powder, and removing the binder from the moldings before sintering them to obtain products.
b) Description of the Prior Art
Injection molding is widely used as a method of forming complicated-shaped metal and ceramic products out of powders. In injection molding, various organic compounds or thermoplastic resins are added to a powder to fluidize it; the powder containing added binder is hot-kneaded, and injection-molded; the binder is removed from the moldings obtained before sintering; and the moldings are sintered into products. In this process, removal of the binders is most important to obtain products without defects. There are two methods conventionally used for removing the binder from powder moldings: the heat decomposition method in which an organic binder is decomposed and gasified by heating, and the dissolution method in which an organic binder is extracted from moldings with an organic solvent.
In the thermal decomposition method, rapid decomposition and gasification of an organic binder raise the internal pressure of the moldings and thereby cause cracks or expansion in them. The decomposition and gasification of a binder therefore must be carried out gradually, and take a long time. Especially for a powder with a small particle size and a large specific surface area to which a larger amount of binder must be added to fluidize it for injection molding, complicated techniques such as heating moldings step by step to higher temperatures are necessary for the removal of the binder, and the removal of the binder takes a longer time. Though addition of volatile substances to binders is thought of, those substances can evaporate during kneading and molding, and make it difficult to reuse the sprue and runner part of moldings.
In the dissolution method, an organic binder is extracted from moldings with an organic solvent, and the pores left by the dissolved out binder serve as passages of decomposed and gasified binder. Therefore, remaining binder is easily removed by heat-decomposition without causing defects in the moldings. In the dissolution method, however, liquid materials such as mineral oils, fatty acid-derived oils, or natural oils are used in large quantitites for a binder. But these oils can separate from the powder during kneading and ooze out of the moldings, so as to make it difficult to store the kneaded powder and moldings stable for a long time. Moreover, oils move to the surface of moldings during storage and can cause cracks or expansion in the moldings during extraction.
A method of dissolving out a plasticizer and a lubricant in an organic binder using hot water is also thought of. In this method, however, the stability of the kneaded powder containing them and the strength of the moldings decrease as the proportion of the plasticizer and the lubricant added increase, whereas cracks and expansion can be caused by heat decomposition and gasification of the binder if their proportion is low.
Disclosed in Japanese Patent Provisional Publication gazette No. 101101/1990 is another dissolution method in which water-soluble resins are used for a binder and they are dissolved out with water. This method has an advantage of using water which is lower in cost and safer to handle than an organic binder. Water, though, is inferior to an organic solvent in the permeating ability into moldings and evaporativity. Therefore, extraction of the binder and drying of moldings after the extraction take a longer time, especially for ceramic or metal powders with a specific surface area of 10 m.sup.2 /g or greater, to which a large quantity of an organic binder must be added. Moreover, when heating moldings to remove the water that has permeated into the moldings, the problems of cracks and expansion can arise as a higher temperature is used. In addition, powders of materials which can rust in contact with water, such as iron and copper, and those of materials which react with water and generate gases, such as ceramic Si.sub.3 N.sub.4 which generates ammonia gas, cannot be used.